Non-magnetic metal mounting frame for pick-up tube targets for preventing microphony



arch 14, 1967 J. E. I. CAIRNS E AL 3,309,552

NON-MAGNETIC METAL MOUNTING FRAME FOR PICK-UP TUBE TARGETS FOR PREVENTING MICROPHONY Fi led May 2, 1963 VIII/[IA m FIG. 1.

FIG.2.

United States Patent dustries Limited, Middlesex, England, a company of Great Britain Filed May 2, 1963, Ser. No. 277,599 Claims priority, application Great Britain, May 4, 1962,

' 17,121/62 Claims. (Cl. 313-89) This invention relates to electrode mountings for pickup tubes, and it relates especially, although not exclusively, to pick-up tubes comprising a double sided target in the form of a dielectric membrane, usually made of glass.

A well-known form of pick-up tube is that known as the image orthicon and this tube has a double sided target in the form of a thin glass membrane. The membrane is supported by a metal frame at an intermediate position within the envelope of the tube, so that one side of the target can receive an electron image from a photo-cathode on one end wall of the tube, and so that the other side of the target can be scanned by the electron beam originating from a gun near the other end of the envelope. A thin mesh is disposed near to and parallel with the target to collect secondary electrons emitted from the target by the electron image. One difficulty which has been encountered in the use of said pick-up tubes, especially if the target area is relatively large, is microphony.

An object of the present invention is to provide an improved television pick-up tube comprising a double sided target in the form of a dielectric membrane, with a view to reducing the difficulty indicated in the preceding para graph.

According to the present invention there is provided an electron discharge device comprising means for producing an electron discharge, which is intended to be magnetically deflected in operation of the device, a thin dielectric target to which said discharge can be directed, and a further electrode parallel to and so close to said target that movement of the target relative to said other electrode can give rise to undesired effects, and wherein said targetis supported by a frame which is substantially uninfluenced by the magnetic field used to deflect the electron discharge so that the risk of vibratory movement of the target rela tive to said other electrode is considerably reduced.

The present invention is based on the experience of the inventors that some of the troublesome microphonyv re- 'ferred to previously appears to be self excited, that is to require no audio frequency or impact excitation. The inventors now believe that the cause of such microphony is due to residual line scan magnetic field in the region of the target. The microphony appears to affect the target because the frame by which the thin dielectric membrane is supported is often a nickel-iron alloy which is magnetic, this alloy being used because it can be sealed to soda-lime glass, from which the membrane is usually made.

In accordance with the invention, therefore the target frame is of non-magnetic metal. A frame of titanium has been found satisfactory in the case of a soda-lime glass membrane. Frames made of other non-magnetic metals may, however, be used provided the target can be sealed thereto. It will be understood that the dielectric membrane of the target may be made of other than soda-lime glass, or may even be made of a membrane of a metallic oxide, for example aluminium oxide.

Magnetisation of the target frame can additionally be reduced, in accordance with the invention, by positioning the target relative to the scanning coils, for a particular design of scanning coils, so that the target is disposed at or near to a position where the scanning magnetic field, especially the line scanning field, is a minimum. It has been found that the residual magnetic field tends to pass through zero and then reverse its sign with increase of distance along the axis of the tube from the scanning coils. Therefore, in accordance with the invention, the target may be positioned as close as possible to the position of minimum magnetic field for the scanning coils.

In order that the present invention may be clearly understood and readily carried into effect, it will now be described with reference to the accompanying drawings, in which:

FIGURE 1 illustrates one example of a television pickup tube in accordance with the invention, the tube being an image orthicon tube, and

FIGURE 2 is a detailed .view of the target assembly of the tube shown in FIGURE 1.

The tube illustrated in FIGURE 1 is generally of conventional construction and comprises a glass envelope 1 having a target assembly 2 located at an intermediate position in the envelope. The target assembly comprises a double sided target which is supported by a metal frame, as will be described later, and one side of the target faces a photo-cathode 3 which is formed on one end wall of the envelope. A mesh 22 is extended before the side or surface of the target which faces the photo-cathode 3. The other side or surface of the target can be scanned by a beam of electrons originating from an electron gun near the other end of the envelope. A field mesh 5 is extended before the scanned side of the target 2. In the image section of the tube there are disposed an image accelerator 6 and a shield electrode 7 to which the target frame is attached whilst in the beam section of the tube there are disposed a persuader electrode 8, wall anode 9 and a decelerator electrode 10. The gun 4 of the tube is surrounded by an electron multiplier 11 for the return beam of electrons, the multiplier being of known construction. When the tube is set up for operation it is disposed within, and co-axial with, a focussing solenoid 12 part of which 12a is designed to focus the electron image in the image section and the remainder of which is designed to focus the scanning beam. An auxiliary focussing coil 13 is also provided in front of the photo-cathode as shown. A scanning coil arrangement denoted generally by the reference 14 is also disposed about the envelope, as indicated, the arrangement comprising a pair of line scanning coils and a pair of field scanning coils, of known construction. A ,u-metal screen 15 surrounds the image section of the screen as indicated to reduce penetration of the magnetic field from the scanning coils into the image section.

In FIGURE 2, in which the target assembly is shown in greater detail, it can be seen that it comprises a double sided target in the form of a thin glass membrane 21 which is sealed to a flanged metal frame 16 and that the membrane 21 is spaced from the metal mesh 22 by means of a mica spacer 17 which has a rectangular aperture. When the target assembly is positioned in the envelope 1 the mesh 22 faces the photo-cathode 3 and acts as a collector of secondary electrons. The mesh 22 is secured to a relatively massive mesh ring 18, which is made of mild steel, the mesh 22 being secured to the ring 18 by a thin securing ring 19. The target frame 16 and the mesh ring 18 are held in relative positions by means of four metal clips 20, three of which can be seen in the drawing, the clips being tightened by means of rivets as indicated. In FIGURE 2, the target assembly is shown before the clips have been tightened, the membrane 21 and the mesh 22 being spaced further apart than when the clips are tightened.

Moreover, in accordance with the invention, the target frame 16 in this example is made of titanium, which is found to be satisfactory for sealing to soda-lime glass from which in this example the membrane 2 is formed. The material from which the mesh ring 18 is made is magnetic in this example as it has been found that provided the target frame 16 is non-magnetic, substantial reduction in microphony is achieved, even when the mesh ring 18 is magnetic, apparently because the mesh ring in this example is sufficiently massive not to be substantially affected by the weak magnetic fields involved. The mesh ring may, however, also be made of non-magnetic material. The mesh 22 and the mesh 5 are made of copper but the mesh may alternatively be made of nickel or silver. Apparently the mesh may be magnetic without introducing microphony because its dimension in the axial direction is negligibly small.

In an alternative form of the invention, the tube illustrated in FIGURE 1 may be modified by arranging the target 2 at a position on the axis of the tube at which the residual magnetic field from the line scanning coil is a minimum or nearly so. It has been found that with a particular designof scanning coil there is a position on the axis of the tube' at which the magnetic field reverses, and the. target is located at this position The operation of the pick-up tube which has been described is conventional, and the following operating voltages may be applied to the'vario-us electrodes, the voltage in each case being given with respect to the Whatwe claim is:' 1. A pick-up tube' adapted for magnetic deflection of the scanningbeam having a target in the form of a dielec- I tric membrane supported by a metal frame, means which in operation of the tube produces a beam of electrons which are directed at one surface of the target, means which in operation of the tube projects an electron image on the other surface of the target, and an electrically conducting mesh disposed adjacent to the target, vibration of said target relative to said mesh being liable to produce spurious variations in the output signal of the tube, wherein said metal frame is constructed of non-magnetic metal thereby to reduce vibration of the target due to the magnetic deflecting field.

2. A pick-up tube having a target in the form of a dielectric membrane supported by a non-magnetic metal frame, means which in operation of the tube produces a beam of electrons which are directed at one surface of the target, deflection 'coil means for magnetically deflecting said electron beam, means which in operation of the tube produce an electron image on the other surface of the target and an electrically conducting mesh disposed adjacent to said target.

3. A pick-up tube according to claim 2 wherein said deflection coil means is such as to produce a minimum deflecting field at a point on the axis of the tube and said target is so disposed that said point lies substantially in said target.

4. A pick-up tube according to claim 1 in which said mesh is supported by a frame of magnetic material which is sufiiciently massive for it to remain substantially uninfluenced by the magnetic deflecting field.

5. A pick-up tube according to claim 2 in which said mesh is supported by a frame of magnetic material which is sutficiently massive for it to remain substantially uninfluenced by the magnetic field produced by the deflection coil means.

References Cited by the Examiner UNITED STATES PATENTS 2,909,687 10/1959 Turk et al. 31367 3,061,752 10/ 1962 Banks 313- 3,067,348 12/1962 Ochs 313-67 JAMES W. LAWRENCE, Primary Examiner. V. LAFRANC HI, Assistant Examiner. 

1. A PICK-UP TUBE ADAPTED FOR MAGNETIC DEFLECTION OF THE SCANNING BEAM HAVING A TARGET IN THE FORM OF A DIELECTRIC MEMBRANE SUPPORTED BY A METAL FRAME, MEANS WHICH IN OPERATION OF THE TUBE PRODUCES A BEAM OF ELECTRONS WHICH ARE DIRECTED AT ONE SURFACE OF THE TARGET, MEANS WHICH IN OPERATION OF THE TUBE PROJECTS AN ELECTRON IMAGE ON THE OTHER SURFACE OF THE TARGET, AND AN ELECTRICALLY CONDUCTING MESH DISPOSED ADJACENT TO THE TARGET, VIBRATION OF SAID TARGET RELATIVE TO SAID MESH BEING LIABLE TO PRODUCE SPURIOUS VARIATIONS IN THE OUTPUT SIGNAL OF THE TUBE, WHEREIN SAID METAL FRAME IS CONSTRUCTED OF NON-MAGNETIC METAL THEREBY TO REDUCE VIBRATION OF THE TARGET DUE TO THE MAGNETIC DEFLECTING FIELD. 